This invention relates to communications devices. In particular, this invention relates to secure communications systems of limited range with low probability of intercept. The disclosure teaches specifically about a novel UV source in the solar blind region of the spectrum which can be used to transmit data at high data rates, e.g. greater than 100 kHz. Short range, compactness, ruggedness, non-line of sight (NLOS), and reduced ambient noise levels are all features of the instant invention.
Solar blind communications systems for covert and NLOS communication have been of interest for several years. Operation in the solar blind spectral region (200 to 300 nm) makes the system immune from natural optical noise (e.g. sunlight). The NLOS and limited range of these systems are the result of signal propagation through the atmosphere via optical scattering. The particularly short range of the instant invention results from increased atmospheric scattering at shorter wavelengths. The applications of such systems range from local battlefield communications, to downloading inertial navigational data to aircraft, to communication among various segments of aircraft or naval vessels.
Most UV based communications (UVCOM) systems to date have relied on the 254 nm emission from atomic mercury. This emission is the basis of all conventional fluorescent lamps. However, fluorescent lamps have phosphor coating on the inner surface of the lamp envelop. Lamps used in the present UVCOM systems are commercially available germicidal lamps. These are mercury rare gas discharges without the phosphor and with a UV transmissive envelope. While these lamps perform well for generating continuous (or 60 Hz) UV, they have certain drawbacks for generating pulsed UV necessary for digital data transmission.
The maximum useful modulation rate using mercury is about 50 kHz. This limitation arises because of the self trapping of the resonance radiation regardless of the method of excitation, i.e. electroded or electrodeless. The present invention presents a system which allows higher modulation rates and thereby increases digital data transmission rates.